Abstract
Previous examination of DNA sequences located 5' to Rhizobium meliloti nif transcription units has shown that extensive sequence homology exists among them. Here we have examined these reiterated sequences for their role in symbiotic gene regulation. Promoter deletion analysis has shown that although an extensive upstream DNA sequence 9160 bp) is required for full heterologous activation of the R. meliloti nifHDK promoter by the Klebsiella pneumoniae nifA protein in Escherichia coli, this region is not required for expression of R. Meliloti nif promoters in root nodules from plants grown under greenhouse conditions. In addition, a minimum functional symbiotic promoter sequence is defined, and a DNA sequence difference affecting regulation of two symbiotic promoters is discussed.